New concrete recycling methods combine two important environmental benefits: they save raw materials and can permanently bind climate-damaging CO2 that would otherwise enter the atmosphere.

Concrete is one of the most important building materials of our time: worldwide concrete production is estimated at 30 billion tons per year. This would theoretically be enough to cover the whole of Austria with a 15 cm layer of concrete.

Given this amount, it would be important to save resources by reusing old concrete. This involves reprocessing construction waste (from demolished concrete, building chippings or masonry demolitions), and the recycled aggregate can then be used for new concrete, producing recycled concrete. However, recycled concrete has poorer mechanical properties than conventional concrete made with natural rock. Experiments show that this disadvantage can be remedied - with a strategy that also has another huge advantage: the properties of recycled concrete can be improved by storing CO 2 in the recycled aggregate . The CO 2 then becomes limestone, strengthens the material and remains permanently stored in the recycled concrete. The Vienna University of Technology is working closely with the Austrian concrete industry on this topic as part of the UP!crete industry project.

Limited recycling options
"Concrete basically has three main components: rock particles - such as sand, gravel or chippings, cement and water," explains Prof. Ildiko Merta from the Institute of Materials Technology, Building Physics and Building Ecology, the scientific director of the UP!crete research project . "The type of rock particles used for the concrete has a decisive influence on the properties of the concrete, such as its strength and durability."

Some of the natural rock can be replaced by recycled aggregates - for example, rubble from demolished concrete buildings. The problem with this, however, is that recycled aggregates are highly porous, they are not quite as dense as natural rock and absorb significantly more water. The strength and durability of the concrete decrease significantly if too large a proportion of recycled aggregates is used in its production.

"Current national standards allow a maximum admixture of 50 percent," says Ildiko Merta. "But our goal would be a recycling share of 80 percent or even higher, because construction waste would generally be available in large quantities."

Extensive experiments were carried out at the Vienna University of Technology and different recycled concrete recipes were developed. Various measures can improve the properties of the concrete - such as coating processes, adding additives or special mechanical processing of the aggregates. This creates high-performance recycled concrete types for various applications.

Concrete as a CO 2 sink
The use of CO 2 has shown great success : "Carbon dioxide is pumped into the recycled aggregate. This leads to carbonation, a chemical reaction that converts the gaseous CO 2 into limestone - similar to the limescale that you see in bathrooms when the water contains a lot of lime," says Ildiko Merta. In this way, the carbon that would otherwise damage the climate in the form of CO 2 in the atmosphere is bound for long periods of time.

With the methods developed and tested at the Vienna University of Technology, two important advantages can be achieved at the same time: On the one hand, the proportion of recycled material used can be increased and the quality of the concrete improved, and on the other hand, the climate can also be helped at the same time because carbon can be permanently removed from the atmosphere.

In her research, Ildiko Merta also attaches great importance to developing technically optimal and economically attractive processes in cooperation with industry that can be used not only in the laboratory but also on a large industrial scale.

"The efficiency of carbonation depends on many parameters, such as pressure, temperature, humidity, or CO2 concentration . However, the optimal conditions are not necessarily those that can be achieved in industrial processes with reasonable effort. One must therefore consider carefully which methods are not only technically possible, but also realistic and efficiently implementable for the economy," says Ildiko Merta.

​https://www.tuwien.at/tu-wien/aktuelles/news/news/mit-recycling-beton-co2-einspeichern